Does anybody know what the threshold is for cash fares reduced by Avios.

For my A350 Madrid Trip the fare was £90.44 and you could save £10 for 1000 Avios and £20 for 2500 Avios.
There fsre now for the same trip is £89.84 but it’s only giving me the option to reduce by £10. I called the Gold Helpdesk and they said the threshold hasn’t been reached but couldn’t tell me the threshold but an £85.32 fare to Newcastle has the ability to save £10 and £20.

Whilst not great value for Avios I’d quite like to save £20 if possible.